1. Field of the Invention
The present invention relates to an optical arrayed waveguide grating type multiplexer/demultiplexer which is employed in the field of optical communication. The present specification is based upon Japanese Patent Application S.N. 2000-188217, and hereby incorporates the subject matter of that application by reference.
2. Background Art
In an optical communication system, in order to transmit a large quantity of information, a wavelength multiplex transmission method is proposed in which light at different wavelengths is multiplexed and transmitted. Further, since light is made up from two polarizations which are perpendicular, a polarization/wavelength multiplex transmission method is also proposed in which, when multiplexing, transmitting and outputting light at different wavelengths, the light is separated for each wavelength and also for each polarization and is outputted. It should be noted that one of the polarizations will hereinafter be referred to as the xe2x80x9cTE polarizationxe2x80x9d and the other as the xe2x80x9cTM polarizationxe2x80x9d.
FIG. 14 is a block diagram which shows an example of a prior art polarization/wavelength multiplex transmission circuit. With this circuit, when multiplexed light of four wavelengths xcex1, xcex2, xcex3, and xcex4 is inputted to an input side optical fiber 1 which is positioned at the left side of the figure, this light is inputted to an input port 2a of a polarization beam splitter 2, is divided into polarizations in this polarization beam splitter 2, and is outputted from output ports 2b and 2c as, respectively, a TE polarization and a TM polarization of the four wavelengths.
Next, the multiplexed light of the TE polarization is inputted via an optical fiber 1 which is connected to the output port 2b to an input port 3a of an optical multiplexer/demultiplexer 3, and is separated into its various wavelengths by this optical multiplexer/demultiplexer 3, so that four TE polarizations of the four wavelengths xcex1, xcex2, xcex3, and xcex4 are output to optical fibers 1 which are connected to its output ports 3b, 3c, 3d and 3e, respectively. On the other hand, the multiplexed light of the TM polarization is also, in the same manner, inputted via an optical fiber 1 which is connected to the output port 2c to an input port 4a of an optical multiplexer/demultiplexer 4, and is separated into its various wavelengths by this optical multiplexer/demultiplexer 4, so that four TM polarizations of the four wavelengths xcex1, xcex2, xcex3, and xcex4 are output to optical fibers 1 which are connected to its output ports 4b, 4c, 4d and 4e, respectively. As a result, it is possible to separate the multiplexed light of the four wavelengths by wavelength and by polarization, into a total of eight parts.
In this manner, with a prior art polarization/wavelength multiplex transmission circuit, they are required both a single polarization beam splitter for separation into polarizations, and two optical multiplexer demultiplexers for separation into different wavelengths. However, since the efficiency of space utilization is deteriorated and the loss is increased when the number of devices which make up the circuit increases, there has been a demand for a technique for building a polarization/wavelength multiplex transmission circuit with a smaller number of devices. In concrete terms, a device would be desirable which could perform separation of polarizations and separation of wavelengths simultaneously.
On the other hand an arrayed waveguide grating type optical multiplexer/demultiplexer is often used as an optical multiplexer/demultiplexer, because its wavelength separation performance is high.
FIG. 15 shows an example of an arrayed waveguide grating type optical multiplexer/demultiplexer: in this figure, an waveguide array 8 is formed by providing a plate shaped cladding layer 6 made from silica based glass upon a silicon substrate 5, and by arranging waveguides 7, 7 . . . made from silica based glass on this cladding layer generally in parallel and in a letter-U configuration. It should be understood that the lengths of these waveguides 7, 7 . . . which make up the waveguide array 8 differ from one another in steps of xcex94L.
At both the ends of this waveguide array 8, i.e. its input side and its output side, respective slab waveguides 9 and 10 are provided. To the input side of this slab waveguide 9 there are provided plural input waveguides 11, 11 . . . in parallel. On the opposite side, to the output side of the slab waveguide 10 there are provided plural input waveguides 12, 12 . . . in parallel. Each of the waveguides 7 through 12, in order for it to propagate light, is made from a material having a refractive index which is higher than that of the cladding layer 6 which is provided around each of the waveguides 7 through 12.
And when multiplexed light consisting of light of plural wavelengths is inputted to one of the input waveguides 11, this multiplexed light is distributed via the slab waveguide 9 between the plurality of waveguides 7, 7 . . . which make up the waveguide array 8 in roughly equal proportions, and is propagated through these waveguides 7, 7 . . . with optical path length differences occurring by xcex94L. And wavelengths are selected by these lights interfering in the slab waveguide 10 on the output side, and these lights of the wavelengths are outputted from the output waveguides 12, 12 . . . respectively.
Moreover, such an arrayed waveguide grating type optical multiplexer/demultiplexer is an optical product which uses a planar waveguide, and just as it is the refractive index differs according to polarization (there is an anisotropy in the refractive index), a polarization dependency exists. Since a planar waveguide, as described above, generally comprises a cladding layer made from silica based glass and a waveguide upon a silicon substrate, during manufacture, due to the difference in coefficient of thermal expansion between silicon and silica based glass, a slight residual stress is engendered in the waveguide in the process of cooling of the cladding layer and the waveguide down from a high temperature to a low temperature, and this residual stress causes a polarization dependency.
Accordingly, just as it is, even at the same wavelength, the focal positions at the output end of the slab waveguide 10 are different for the different polarizations.
Since this characteristic is an inconvenience when separating the wavelengths, in the prior art, as shown in FIG. 15, a half wavelength plate 13 is inserted in the center of the waveguide array 8, and thereby characteristics which do not depend upon polarization are obtained.
Or the method is employed of setting the position of the input end of each of the respective output waveguides 12, 12 . . . of the slab waveguide 10 of the output side at the central point between the focusing positions of the two polarizations which make up the light of the wavelength which is distributed to this output waveguide 12, so as to output these two polarizations from a single output waveguide 12.
On the other hand, a proposal has been made to use an arrayed waveguide grating type optical multiplexer/demultiplexer as a polarization beam splitter by taking advantage of this polarization dependence.
However, no proposal has ever yet been made for an arrayed waveguide grating type optical multiplexer/demultiplexer which simultaneously performs wavelength separation and polarization separation.
The present invention has as its subject the provision of an art which is capable of constructing a polarization/wavelength multiplex transmission circuit from a small number of comprised devices. In concrete terms, its objective is to provide an optical multiplexer/demultiplexer which is capable of simultaneously performing separation by wavelength and separation by polarization. Furthermore, another of its objectives is to provide an optical multiplexer/demultiplexer of the arrayed waveguide grating type which is capable of simultaneously performing separation by wavelength and separation by polarization.
In order to fulfil this objective, the present invention proposes an arrayed waveguide grating type optical multiplexer/demultiplexer in which a waveguide, comprising a waveguide array in which plural waveguides of different lengths are arranged in parallel, a first slab waveguide and a second slab waveguide which are provided at opposite ends of said waveguide array, an input/output waveguide which is provided on the outer side of said first slab waveguide, and plural input/output waveguides which are provided on the outer side of said second slab waveguide, is provided in a cladding layer, characterized in that, when multiplexed light consisting of lights of plural different wavelengths is inputted into said input/output waveguide which is provided at the outer side of said first slab waveguide, the lights which have been separated by wavelength and by polarization are outputted from said plurality of input/output waveguides which are provided at the outer side of said second slab waveguide.
The following type of benefits are obtained by the present invention.
It is possible to provide an optical multiplexer/demultiplexer which can perform separation and combination of lights of different wavelengths and polarizations simultaneously.
Because of this, instead of using a polarization beam splitter and two optical multiplexer/demultiplexers, it is possible to build a polarization/wavelength multiplex transmission circuit with a single optical multiplexer/demultiplexer, so that it is possible to envisage economy of space and reduction of losses.